add a 'page does not exist' page

[RaimundSchramm]

• lack of knowledge: this is default behaviour and already in place
• in production we have working 404, 422 and 500 static pages that are served by Rails
• in development though the option is to always see the trace for debugging, so these pages don't get rendered by default (can be set in development.rb)
• decision is to leave this in place and maybe design a more dynamic differently styled page later

• great write-up here
• and here